Now that you have your foundation, you most likley will want to build on top of that. The remainder of this tutorial is purely creative and specific steps are difficult to outline. I will merely provide you with a few pointers and examples of what I do to create the borders.

If you wish to keep a sharp edge on your border, leave it as is. If you want a softer, corrosion-like feel to it, you will need to blur the entire layer once (Filter > Blur > Blur). For all of the example images you will see below, I will apply the blur filter.

If you want to take the corrosion even further, you will want to make use of the brushes that you downloaded. Using your eraser tool, select one of the new brushes and randomly click on the grunge border. This will “eat” away at the grunge border. Depending on the texture of the brush, it will not completely erase the border, but leave some rather nifty effects. =] Below is an example of what I mean:

Next step is to add some additional pieces to the border, be that some grunge text or other rough elements. Try experimenting with various text, brushes, lines, etc. Once you are happy with what you have, merge the new layer into the grunge border layer (Ctrl + E). Below is an example:

There is not a right or wrong way to go about this tutorial, much like most of the tutorials I have seen. You are only limited by your own creativity. There are several ways you can tweak this tutorial for a different outcome. For instance, you could change the settings when creating the initial grunge_border.psd file. Or you could add, remove, edit the various components of the border.

Another key point to mention that the border is only a subcomponent of the final image. Try not to let the border take over your work.